Recent advances in the therapy of CML have resulted in a very high remission rate, including hematologic and cytogenetic responses in the vast majority of patients. However, minimal residual disease can be detected in most patients by PCR monitoring. Levels of Bcr/Abl mRNA detected by reverse transcriptase PCR probably underestimate the residual leukemia burden, as transcriptionally inactive, quiescent CML cells cannot be detected by this technique. Bcr/Abl signaling negatively regulates the expression ofthe chemokine receptor CXCR4 on CML cells. CXCR4 is the receptor for SDF-1, a major chemokine produced by the bone marrow microenvironment. Downregulation of CXCR4 results in release of CML cells/progenitor cells from the bone marrow into the circulation and may explain the characteristic leucocytosis of CML. We have recently demonstrated that Bcr/Abl kinase inhibitors such as imatinib upregulate CXCR4 expression and may therefore favor the survival of CML progenitor cells in the bone marrow niche. Pharmacological inhibition of CXCR4 by a specific small molecule inhibitor (AMDSIOO/Plerixafor) in leukemia patients resulted in striking release of malignant cells from the bone marrow into the circulation and in the CXCR4-mediated inhibition of pro-survival signaling in the leukemic cells, independent of their mobilization. Our in vitro and in vivo data in myeloid leukemias demonstrated that disruption ofthe CXCR4/SDF1 axis results in pronounced sensitization to signal transduction inhibitors and chemotherapy thus overcoming microenvironment-mediated resistance. A clinical trial is ongoing and has provided proof-of-concept, i.e. that CXCR4 inhibition results in preferential mobilization of Ph+ leukemia cells. In addition, integrins mediate anchoring of leukemic cells in their microenvironment and inhibition of VLA-4 resulted in disruption of leukemia/stroma cell interactions and chemosensitization. We therefore propose: Specific Aim 1. To test the hypothesis that inhibition of CXCR4 and/or VLA-4 results in disruption of CML/stroma interactions and thus reverses microenvironment-mediated protection of CML progenitor cells to tyrosine kinase inhibitors. Specific Aim 2. To test the hypothesis that non-genotoxic activation of p53 by Nutlin-3a in chronic phase and blast crisis CML disrupts leukemia/stroma interactions via SDF-1/CXCR4 inhibition. Specific Aim 3. To investigate the potential of combined tyrosine kinase and Bcl-2 inhibition on the survival of quiescent CML progenitor cells in the bone marrow microenvironment in vitro and in vivo. We expect these studies to improve our understanding of the complex interactions between CML cells and their microenvironment and provide mechanism-based rationale for eliminating residual CML progenitor cells.